There is a demand for improving a transmission rate of a wireless communication for a purpose of satisfying increased traffic, in a field of a mobile communication system such as a mobile phone. NTT DOCOMO technical journal, Vol. 18, No. 2, p 12 to p 21 describes Carrier Aggregation (CA) technology for communicating with use of a plurality of carriers having a different frequency band as a method of improving the transmission rate. A combination of 800 MHz and 2 GHz may be used as the frequency band used for the Carrier Aggregation. A combination of other various frequency bands may be used.
In a high frequency circuit for transmitting and receiving in the CA, a duplexer allowing passage of a signal in a specific pass band is provided between a common antenna terminal and each of internal terminals (a receiving terminal or a transmitting terminal). A plurality of duplexers are provided so as to cover different frequency bands. A diplexer is provided between the antenna terminal and each of the duplexers. The diplexer has characteristics in which a low pass filter and a high pass filter are combined, and acts as a filter for restraining mixing of a signal (for example, 2 GHz) in a signal path (for example, 800 MHz). The diplexer may be a lumped constant type in which an inductor and a capacitor are combined or a distributed constant type in which a λ/4 line is provided.
In a high frequency wave circuit for transmitting and receiving in the conventional CA, a diplexer is provided between an antenna terminal and each duplexer. However, it is difficult to downsize the diplexer, and the diplexer has relatively large insertion loss. Skirt characteristics of an attenuation curve of the diplexer are not precipitous. Sufficient signal suppression may not be secured in the frequency bands, when the frequency bands used for the Carrier Aggregation are close to each other.